The only correct answer to this is: it depends. Sometimes teenagers go on 'vegetarian' or 'vegan' diets, partly out of a moral compunction, partly out of a desire to lose weight- but with little planning or preparation. It is, in fact, possible to devise a 'vegan' diet comprised entirely of junk food!

In general, however, a vegan diet offers the promise of excellent nutrition. Partly that's because plant-based diets are a good idea, and partly because vegans tend to be especially conscious about health and food choices, and choose very carefully.

What we do not have to date is any head-to-head comparison study that pits variations on the theme of 'optimal' eating against one another. Is an optimal vegan diet better or worse than a diet that is mostly plant-based, but includes fish? What about the Mediterranean diet? How about a 'paleolithic' style diet? The direct comparisons have not been made, and we really don't know. We do know that our species has been omnivorous from the start, and the species that preceded ours were as well- for a very long back in history.

We also know that most people living in the U.S. and other industrialized countries eat too much meat for their health, or that of the planet. Michael Pollan's advice stops short of veganism, but points in the right direction: eat food, not too much, mostly plants. For six good reasons why you should eat more plants, if not only plants, see Michael Jacobson's 'Six Arguments for a Greener Diet."

If inclined to eat ONLY plants, approach it with thought and preparation- and it is very likely to promote your health. I recommend the excellent book by Davis and Melina, "Becoming Vegan," as a step-by-step guide to doing vegan well.